Anatomical Study on Wax apple (Syzygium samarangense) Roots under Long-term Water-logged Conditions

Abstract

Tropical regions are prone to seasonal rains that leave the soil water-logged for long periods. Studying the impact of long-term water-logging on root development is important for fruit tree cultivation in the tropics. Myrtaceae roots are known to develop a protective tissue called polyderm. Among many Myrtaceae fruit trees, we observed that wax apple roots bear the most number of polyderm layers. How do polyderm-bearing roots respond to water-logged condition? In this anatomical study, young and older root tissues of three-year-old wax apple trees grown in normal and long-term water-logged conditions were compared. Root specimens sectioned by freehand or with an ultramicrotome were stained and observed under a fluorescent or optical microscope. Under normal condition, up to 5 layers of polyderm layers, composed of alternating endodermis-like cell layers and thick-walled cell layers with lignin and suberin accumulation, were observed whereas in water-logged condition, polyderm layers got restricted to 2-3 layers. In young water-logged roots, lignin-like accumulation in the inner cortical cell wall was fewer than in normal roots. Aerenchyma developed dramatically in cortical area of roots grown in water-logged condition signaling the hypoxic nature of the soil environment. Decrease of polyderm layers in water-logged condition indicates the insignificance of adding new cell layers with modified cell walls beyond the endodermis. Further experiments must be carried out in varying degrees and periods of water-logging to understand the mechanism controlling cell wall modification in root tissues and to clarify the role of structural changes and its possible long run effects.